This invention relates to a method, implemented by computer, for predicting the aerodynamic performance of an aircraft or of a portion of an aircraft. Throughout the following, the phrase “aircraft element” designates, in the particular instance, a portion of an aircraft or an aircraft in its entirety.
The invention applies to aircraft elements having at least one break, and more particularly—but not exclusively—to aircraft elements comprising at least one moving surface, such as a flap, wing flap, slotted flap, Fowler flap, slat, Kruger slat, retractable slotted flap, aileron, spoiler, rudder, elevator. . . .
Predicting the aerodynamic performance of an aircraft element, and in particular of an aircraft element with moving surface(s), is essential in particular in order to make it possible to:                calculate the loads sustained by the aircraft element and size the structure thereof accordingly and in optimized manner,        size each moving surface as well as the corresponding actuator,        work out a flight mechanics model of the aircraft, develop a flight simulator and flying rules,        determine the operating features of the aircraft, set forth in operating-features and flight manuals.        
Any method for predicting the aerodynamic performance of an aircraft element is intended essentially to make it possible to work out an aerodynamic model and to provide estimations of the aerodynamic data that characterize the aerodynamic performance of the aircraft or of the aircraft element. The phrase “aerodynamic data” used here encompasses the following data:                the aerodynamic coefficients of the aircraft or, as the case may be, of a portion thereof; these coefficients are non-dimensional parameters used for quantifying the forces or moments exerted by the air in motion on a portion or the entirety of the aircraft; among these coefficients there may be cited, for example, the lift coefficient CZ, the drag coefficient CX. the drift or side force coefficient CY, the roll moment coefficient CL, the pitch moment coefficient CM, the yaw moment coefficient CN of the aircraft or, as the case may be, of a portion thereof,        other aerodynamic parameters such as the lift, drag, drift, roll, pitch and yaw efficiencies of each moving surface of the aircraft element; the lift (respectively drag, drift, etc.) efficiency of a moving surface expresses the impact of the total deflection of this moving surface on the lift (respectively drag, drift, etc.) coefficient of the aircraft or, as the case may be, of a portion thereof; these efficiencies have an effect on the flight characteristics of the aircraft;        the load distribution on the aircraft element, or even any pressure coefficient making it possible to quantify locally the pressure exerted by the air in motion; the stresses sustained locally by the aircraft element determine the sizing thereof,        the hinge moment of each moving surface of the aircraft element, which corresponds to the torque applied to the hinge during deflection of the moving surface; the hinge moments are necessary to the sizing of the actuators for the moving surfaces.        
It is to be noted that the value of an aerodynamic datum depends on a set of operational and environmental conditions, including the angle of incidence, the angle of deflection of each moving surface, the speed of the air flow in relation to the aircraft, the temperature of the air, the pressure. . . . These conditions also may be defined with the aid of parameters such as: the Reynolds number Re (which represents the relationship between the forces of inertia and the viscous forces); the Mach number Ma (which represents the ratio between the speed of the air flow in relation to the aircraft to the speed of sound); etc.
Various means may be used to work out an aerodynamic model, including semi-empirical methods, digital simulation methods, wind-tunnel tests, flight tests, combinations of the aforementioned means. These means are the subject of ongoing research with a view to their improvement.
The known methods for predicting the aerodynamic performance of an aircraft element by digital simulation generally consist essentially in:                creating a digital object, known as digital form, representative of the form that the aircraft element assumes under predetermined (in particular operational) conditions,        making a meshing around the said digital form,        carrying out digital aerodynamic simulation computations on the basis of predetermined conditions studied with the aid of an aerodynamic computation code, that is to say, a code capable of resolving fluid mechanics equations, these computations providing certain properties (including the pressure) of the fluid around the aircraft element.        determining aerodynamic data that characterize the aerodynamic performance of the aircraft or of the aircraft element, from the results of the preceding computations.        
The determined aerodynamic data for an aircraft must be updated regularly during the various design and development phases of the aircraft, so as to follow the evolutions in geometry of the aircraft and to provide more and more precise estimations of these data.
The methods of prediction by digital simulation used also must be updated regularly during the development of the aircraft, in order to take into account not only the geometric evolutions of the aircraft but also the latest technical advances achieved in terms of digital simulation. Among these advances, there may be cited the improvement of certain physical models such as the one for turbulence, the advent of new digital techniques including the meshing technique known under the name “Chimera,” the ongoing increase in the capacity of computation means. . . .
The form of the aircraft element to be studied may make modeling thereof particularly complex. Such is the case for an aircraft element—for example a wing or other wing-group element of an aircraft—comprising one or more high-lift devices or other moving surfaces. The step of meshing of such an aircraft element is particularly delicate. The difficulty is accentuated by the fact that the high-lift devices are both detached from and close to the rest of the wing group. In the earlier known methods, this meshing step consists either in generating an unstructured meshing, or in using the Chimera technique to obtain a structured meshing.
It is recalled that an unstructured meshing generally is made up (in 3D) of tetrahedrons, prisms, hexahedrons and pyramids, assembled in any manner. The topology of such a meshing is arbitrary.
A structured meshing is a meshing that may be generated by reproducing a basic mesh. In such a meshing, any node may be identified (in 3D) by a triplet.
The generation of an unstructured meshing is more automatic, and consequently often faster, than that of a structured meshing, which presupposes the creation of a topology and for this purpose generally requires a human expertise. On the other hand, the generation of a unstructured meshing demands a more extensive description of its elements and a greater random-access memory capacity than those required in the case of a structured meshing. Moreover, the computations carried out on an unstructured meshing generally are longer.
Furthermore, the structured meshings generally provide more precise results than the unstructured meshings, and for various reasons: the modification of the meshing and the local control of the quality of same are easier in structured form; meshing of the boundary layers and of the wakes is of better quality; the digital diagrams (manner in which the equations are introduced into the computation code, discretisation . . . ) generally are more precise in the case of a structured computation code (using a structured meshing). It so happens that the precision with which the aerodynamic data are estimated has a direct impact on the weight of the aircraft. Thus for example, insufficiently precise results make it necessary, for safety, to oversize the moving surfaces in order to guarantee a desired level of maneuverability of the aircraft; this oversizing necessitates the use of more powerful and therefore heavier actuators.
For all these reasons, it seems desirable, as regards predicting the aerodynamic performance of an aircraft element, to have methods using structured meshings. Considering the respective advantages and drawbacks of structured and unstructured meshings, it moreover is worthwhile to carry out both digital aerodynamic simulations on the basis of unstructured meshings and simulations on the basis of structured meshings. Also and above all, the development of these two approaches provides a redundancy that makes it possible to detect and eliminate possible errors and to evaluate the precision of the results obtained (if the results obtained with a structured meshing and with an unstructured meshing are close, the precision is high; conversely, far-apart results express a low precision).
The known methods for predicting aerodynamic performance of an aircraft element using unstructured meshings are relatively satisfactory.
On the other hand, the making of structured meshings around an aircraft element with moving surface(s) poses a problem. The only solution known to date that makes it possible to mesh such an element in complex configurations (for example deflected high-lift devices) consists in using the Chimera technique, also called “overset grid method.” This technique consists in constructing sub-domains that partially overlap in order to avoid the use of an overall meshing. The boundary conditions must be respected at the physical boundaries between the different sub-domains, which more often than not is obtained through an interpolation.
The known methods using the Chimera technique necessitate substantial computation times and a cumbersome and complex implementation. The making of the meshing and the preparation and execution of the subsequent computations remain very difficult, or even impossible in the most complex configurations (deflected high-lift devices, engine installations represented, flap fairings represented . . . ). For the latter, recourse to an unstructured meshing remains the only possibility for carrying out a digital simulation.